Propeller wakes under different loading conditions obtained by the improved delayed detached eddy simulation method were studied based on the flow decomposition technique. The sparsity-promoting dynamic mode decomposition was used to study the flow physics in the wake of a propeller, with particular emphasis placed on identifying the underlying temporal and spatial scales that play important roles in the onset of propeller wake instabilities. The morphology of flow structures of different modes selected by the sparsity-promoting algorithm at different frequencies characterizes the instability process of the wake system. It shows that the circumferential diffusion of tip vortex structures promotes the approaching of adjacent tip vortices, enhancing the interaction of the vortex pairs, which plays an important role in the instability triggering mechanism of the propeller wake, especially the mutual inductance between neighboring tip vortices. The present study further extends knowledge of propeller wake instability inception mechanisms under different loading conditions.

This work investigates the flow physics in propeller wakes to better understand how propeller wakes evolve under different inflow conditions from near field to far field. A rotating propeller is numerically modeled by using a dynamic overset technique that involves the improved delayed detached-eddy simulation method. To validate the numerical approach, its results are compared against experimentally determined thrust and torque coefficients and flow fields. The results show that, compared with uniform inflow, turbulent inflow significantly modifies the morphology of the vortex system behind the propeller. Under turbulent-inflow conditions, turbulent structures appear around the boundary layer of the propeller blades and interact with the boundary layer flow of the propeller blades, leading to instability and diffusion of primary tip vortices shed by the blade tips. Multiple local pairing in the circumferential direction leads to the rapid breakdown of the tip vortex system, accompanied by the generation of numerous secondary vortex structures. Tip vortices quickly lose coherence in the middle field and far field and tend to be homogeneously distributed when there is inflow turbulence. The present study gives a deeper insight into the flow physics driving the tip vortex pairing process for a propeller operating under uniform- and turbulent-inflow conditions.

The wake instabilities of a propeller operating under turbulent-inflow conditions were studied by the improved delayed detached eddy simulation method on an unstructured mesh consisting of almost 82.5 × 106 cells, capturing propeller wakes extending to the downstream distance of 9 D (where D is the propeller diameter). Two turbulent-inflow cases with the turbulence intensity of 5% and 20% were considered. The mean loads and phase-averaged flow field show good agreement with experiments. As the propeller blade interacts with the turbulent inflow, a wide peak extending approximately ±10 Hz in the power spectral density of the time histories of the thrust and torque coefficient. Simulation results reveal wake instability mechanisms of the propeller operating under different turbulent-inflow conditions. The turbulence added to the inlet boundary interacts with the tip vortices, which accelerates the destabilization processes of the tip vortex system from two aspects. First, the interaction between the inflow turbulence and the tip vortex promotes the diffusion of tip vortices. Second, the interaction between the inflow turbulence and the tip vortices magnifies the instability motion of the tip vortex. The wake vortex system of the high-turbulence inflow condition loses its stability after 2.2 D downstream, while the initial instability behaviors for the low-turbulence inflow condition are observed at the location of 3.4 D downstream. The present study presents a deeper insight into the flow physics driving the tip vortex pairing process for a propeller operating under turbulent-inflow conditions.

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